Add comprehensive fuzz testing infrastructure with 6 new targets

- Fix numpy.c: tautology bug (|| → &&), infinite loop, and missing
  sqlite3_vec_numpy_init call
- Replace tests/fuzz/Makefile: auto-detect clang, add UBSAN, macOS
  ld_classic workaround, generic build rules for all 10 targets
- Add 6 new fuzz targets: shadow-corrupt (corrupted shadow tables),
  vec0-operations (INSERT/DELETE/query sequences), scalar-functions
  (all 18 SQL scalar functions), vec0-create-full (CREATE + lifecycle),
  metadata-columns (metadata/auxiliary columns), vec-each (vec_each TVF)
- Add seed corpora for shadow-corrupt, vec0-operations, exec, and json
- Add fuzz-build/fuzz-quick/fuzz-long targets to root Makefile

All 10 targets verified building and running on macOS ARM (Apple Silicon).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/fuzz/vec-each.c

@@ -0,0 +1,32 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "sqlite-vec.h"
+#include "sqlite3.h"
+#include <assert.h>
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  int rc;
+  sqlite3 *db;
+  sqlite3_stmt *stmt = NULL;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  rc = sqlite3_prepare_v2(db,
+    "SELECT * FROM vec_each(?)", -1, &stmt, NULL);
+  assert(rc == SQLITE_OK);
+
+  sqlite3_bind_blob(stmt, 1, data, (int)size, SQLITE_STATIC);
+  while (sqlite3_step(stmt) == SQLITE_ROW) {
+    // Consume all rows — just exercise the iteration path
+  }
+
+  sqlite3_finalize(stmt);
+  sqlite3_close(db);
+  return 0;
+}